From the experts:
Is Merrin a real name or a melodic invention? Singer Bo Bice brought this merry name to the public's attention when he chose it for his daughter, but most reliable baby name sources don't list it. But there is a Cornish saint's name Merryn, thought to be a variation of the Irish Mirin or Mirren, and so we're going to call Merrin another twist on that ancient group of names.
Or you might think of Merrin as a spin on the more modern Merry and Marin. Or one of the modernized Irish baby names that's traveled internationally.
Whatever its derivation, Merrin is both usual and accessible, an excellent combination, if a bit synthetic-feeling.